Walden Data Aids Climate Science

Dressed in a green T-shirt, brown zip-up hoodie, jeans, and sneakers, the only hint that Charles C. Davis is a biologist comes from the stitching of a plant on his jacket arm. Around his office lies a Chopin CD, a bobblehead tiger (given to him by his mother), and an Indian decorative cloth that his friend gave him when Davis was planning to become an engineer.

But a closer inspection reveals that his walls are covered with depictions of Vincent Van Gogh and Gustav Klimt trees, and next to his metal filing cabinet stands one made out of oak wood. The office, it turns out, also overlooks the trees surrounding the Harvard University Herbaria, home to more than five million plant specimens.

But Davis’ research as a biologist has led him beyond the Cambridge plant collection to a far more storied one just a dozen miles away. For the past few years, Davis and colleagues from Harvard and Boston University have been perusing the notebooks of the famous naturalist and philosopher Henry David Thoreau, using his notes about his sanctuary at Walden Pond to uncover the drastic effects of climate change.


The research began when BU biology professor Richard B. Primack ’72 was revising his textbook on conservation biology and his section on climate change “kept on getting bigger and bigger.”

Yet all the examples about global warming were of places like Canada, Costa Rica, or Europe. Primack said he wanted to prove that global warming was affecting the United States, and so he embarked on a search for evidence through historical records and botanical society meetings.

With his graduate student, Abraham J. Miller-Rushing, Primack stumbled upon Thoreau’s observations of changes in plant flowering times and species occurrences over time.

“It became the gold mine,” Primack said. “What was great was that Thoreau was so famous and that his records were the oldest we found in the United States.”

As Primack and Miller-Rushing conducted their fieldwork over several years, the two sought to publish their Walden Pond evidence in an ecology journal.

But two years ago, the pair was presenting their work at the Harvard Herbarium when, in the middle of the talk, Davis popped his hand in the air.

The Harvard evolutionary biologist approached the BU researchers about conducting a combined analysis of their work: connecting the changes in flowering times with a decrease in biodiversity.


Together with his graduate students, Charlie G. Willis and Brad R. Ruhfel, Davis compiled an evolutionary tree of the entire community of flora that had existed in the Concord area in the mid-19th century.

“Using phylogenies to think about interesting patterns of bioevolution and global [climate] change just seemed like a perfect avenue to think about this pattern of species loss using a novel evolutionary perspective,” Davis said.

Primack and Miller-Rushing had observed that the plants around Walden Pond were producing flowers on average more than a week earlier than they were in Thoreau’s time, when temperatures were 4.3 degrees Fahrenheit lower.

The shift in flowering times, however, was not uniform—some species groups were flowering more than three weeks earlier, while others were flowering “like clockwork around mid-May,” Davis said.

Applying these data to an evolutionary perspective, the researcher--s found that the species that adjusted to the changing climate survived, while the “clockwork” plants had declined in number.

“The real downer about this all is that the groups that are being hardest hit are our most cherished temperate flowering species: orchids, buttercups, roses, dogwoods, violets,” Davis said. “These are the kind of species that people go out on botanical forays to see, and now they can’t see them.”

Davis said that about one-quarter of the plants Thoreau observed in his notebooks have become extinct, and that 36 percent now are in such low abundance that they are “hanging by a thread.”

Willis, one of the graduate students, called the study’s findings “pretty heartbreaking.”

“It’s cool how we see these broad patterns of evolutionary change,” he said, “but it’s also sad how these plants are dying.”

Ruhfel, the other graduate student, noted that the exceptionally long span of the Walden Pond data is what allowed them to reach such striking conclusions.

“It’s 150 years of what has gone on in Concord, and it’s rare to have this kind of data,” he said.


While Ruhfel acknowledged that Thoreau’s historical stature has contributed to the public reaction to their work, he maintains that the scientific merit of the study does not depend on the origin of the data.

“The article would have been published anyway if it wasn’t Thoreau’s data and it was just some guy’s,” he said, “but the fact that he was attached to it makes it more accessible to more people.”

Davis agreed, saying that, “Thoreau holds a very special place in many of our hearts.”

“It really does bring the issue of climate change to our very own backyard,” he said. “People are definitely buzzed about it.”

Primack said that the headline-grabbing nature of their work was evident to them and that their work was designed to garner attention from the start.

“We didn’t want to just do the best scientific research, but have the most impact as possible,” Primack said. “We wanted to convince the American public that climate change was a reality.”

Davis said that the study would have implications that go far beyond Walden Pond. The patterning of species loss at Walden seems to extend from one habitat type to another, from wetlands to deciduous forests, he said.

In the conclusion of their published article, the researchers draw parallels between the species loss in Concord and the mass extinctions of the past.

Davis said that by studying their similarities, the two sets of events may prove to be illuminating for explaining the reasons behind the loss of biodiversity over time.

Some scientists call the modern era the sixth mass extinction that is taking place, Davis said, and so the study “gives us a very strong shot in the arm about thinking in an evolutionary way about which species are going to decline to climate change.”


This study has not only taken a novel approach at which scientists look at climate change, but it has also created a unique story of scientific collaboration across disparate disciplines.

All the scientists involved in the study said they were amazed how the study even came together through the collaboration between the two research groups.

“Without working together we would not have been able to do it at all,” Primack said. “This is an extremely productive interaction that brought together results that neither [research group] would have anticipated.”

Davis said that academics talk a lot about the goal of integrative and interdisciplinary work, and that this is an instance of an actual project coming to fruition.

“It’s beautiful that the system is so close and that we’ve developed this synergistic approach bridging ecology and evolutionary biology,” Davis said.

He added that for a number of years, evolutionary biologists have stood on the sidelines when it comes to addressing how species are responding to climate change, and that their research sends a different message.

“I don’t want to be pompous here, but I think it’s the dawning of a new age about species loss and climate change,” Davis said.